Electrowetting module

ABSTRACT

An electrowetting module ( 20 ) comprises a fluid chamber ( 8 ) which contains a first fluid (A) and a second fluid (B), which are separated by an interface ( 14 ) , and means ( 16, 17 ) to exert a force on at least one of the fluids to change the position and/or shape of the interface. By providing the second fluid body with a dissolved or mixed compound, being insoluble in or immiscible with the first fluid body, and/or the first fluid body comprises a dissolved or mixed compound being insoluble in or immiscible with the second fluid body, in amounts sufficient for lowering the freezing point of the respective fluids to below −20° C., the performance of the module can be improved so that the module can be used at low temperatures.

The invention relates to an electrowetting module, comprising a fluidchamber, which contains at least a first body of a first conductingand/or polar fluid and a second body of a second non-conducting and/ornon-polar fluid, said bodies being separated by an interface, and meansfor exerting a force on at least one of the bodies to change theposition and/or shape of the interface.

It is observed that wetting techniques make it possible to manipulate avolume of a fluid along a predetermined path. With these techniques, thesurface tension of said volume is locally altered (usually reduces),causing the volume to flow in the direction of its lowest surfacetension.

Further, it is observed that a fluid is a substance that alters itsshape in response to any force, and includes gases, vapors, liquids andmixtures of solids and liquids, capable of flow.

The term “wettability” of a surface by a certain fluid gives anindication of the ease with which said fluid may wet said specificsurface, which may for instance depend on the nature of and/or theelectric potential across said surface. If a surface has a “highwettability” by a specific fluid, this indicates that a droplet of saidfluid in contact with said surface will have a rather expanded shape,with a relatively large contact area and a relatively small contactangle, usually less than about 90°. “Low wettability” indicates that thedroplet in contact with said surface will have a rather contractedshape, with a relatively small contact area and a relatively largecontact angle, usually exceeding about 90°.

The term “wetting” is understood to encompass all techniques causing thesurface tension of a volume, e.g. a droplet of a specific fluid to belocally varied, so as to influence the wetting behavior of said fluidwith respect to a specific surface.

In modules wherein use is made of the wettability phenomenon, it isnecessary that the two fluids have desired properties, for example:densities as close as possible; low melting points; adapted viscosity;good electrowetting behavior; non poisonous; and, in case of an opticalmodule, indices of refraction of a certain predetermined difference.

An example of such an optical module is an electrowetting-based lens,also called an electrowetting lens, of which the focal distance can bechanged. In an electrowetting lens the interface between the two fluidbodies is a meniscus. In such a module the first fluid body is anelectrically conducting and/or polar liquid and the second fluid body isan electrically non-conducting and/or non-polar liquid. The first liquidis, for example salted water and the second liquid is, for example anorganic non-polar, water-inmiscible liquid such as decane and siliconeoil. The electrowetting optical module is provided with means forexerting an electrical force by means of which the shape and or theposition of the meniscus can be shaped. Other examples of theelectrowetting optical module are zoom lenses, diaphragms, diffractiongratings, filters and beam deflectors. Embodiments of these modules aredescribed in PCT patent application no. IB03/00222 and in Europeanpatent applications nos. 020789309.2, 02080387.0 and 02080060.3. Theelectrowetting optical modules are very compact and may therefore beused with much advantage in devices, like optical disc scanning devices,mini cameras for a/o mobile phones, displays etc.

In optical electrowetting modules, the temperature range in which themodule can be used depends greatly on the freezing point of the fluidbodies used. There is a growing demand for optical electrowettingmodules, which will still work at temperatures well below 0° C.,preferably at temperatures at approximately −20° C. Furthermore, thestorage temperature should be as low as −40° C. To prevent damage to thehousing, the liquids should preferably not solidify above thistemperature.

The known aqueous solutions, and some non-conducting liquids, do notmeet this requirement.

The problem of lowering the freezing point of the fluid body belowapproximately −20° C. remains thus to be solved.

It is an object of the invention to provide an electrowetting module asdefined in the opening paragraph, which module, if used as an opticalmodule, allows varying the optical power over a large range and canstill be used at a low temperature. The electrowetting module ischaracterized in that the second fluid body comprises a dissolved ormixed compound, being insoluble in or immiscible with the first fluidbody, and/or the first fluid body comprises a dissolved or mixedcompound being insoluble in or immiscible with the second fluid body,said amounts of compounds being sufficient for lowering the freezingpoint of the respective fluids to below −20° C. The freezing point ispreferably lowered to below −30° C. Even more preferred the freezingpoint should be lowered to −40° C. in order to allow storage of thedevice at all environmental circumstances. Experiments have shown that asolution having a concentration of 4 M is required to lower the freezingpoint of the most suitable fluids to −20° C., as will be explainedhereafter.

It is remarked that an electrowetting lens with a first fluid bodyconsisting of salted water is disclosed for example by B. Berge and J.Peseux in Eur. Phys. J.E3, 159–163 (2000). Said fluid body consists morespecifically of a few percent of Na₂SO₄ in water. Such an amount ofdissolved salt is nevertheless too low to decrease the freezing pointsubstantially.

Inorganic salts containing cations having an atomic weight below 50 u,preferably below 40 u, are very suitable for use in the conductive fluidof the electowetting module of the invention, because these compoundshave the additional advantage that the density as well as the refractiveindex of the solution will still have a low value. Preferred compoundsare chlorine salts, more specifically lithium chloride.

It is observed that it is known from JP-2001013306 A to use an aqueous10 wt. % solution of sodium chloride as the conductive liquid in anelectrowetting lens. Such an amount of salt corresponds with a 1.83 Msolution; it is only possible to obtain a freezing point depression ofup to −6.5 C with such a solution.

Preferred compounds to be used in the second, non-polar fluid body forlowering the freezing point of said body are claimed in claim 7.

A module as indicated above may be configured as an optical component,the first and said second fluid body having different refractiveindices. In such an optical module the salt added to the firstconducting fluid has a refractive index difference maintaining effect.

In such a module the first fluid body may be electrically conductingand/or polar, and the second fluid body may be electricallynon-conducting and/or non-polar and the module may be provided withmeans for exerting an electrical or mechanical force to change theposition and/or shape of the meniscus-shaped interface.

The difference in refractive index is from 0.01 to 0.3, preferably from0.1 to 0.2; the refractive index of one of the fluid bodies beinggreater than 1.4, preferably greater than 1.5.

Preferably the first and second fluid bodies show a substantiallysimilar density.

The module may also be provided with means for exerting a pressure tochange the position and/or shape of the interface.

These and other aspects of the invention will be apparent from andelucidated by way of non-limitative example with reference to theembodiments described hereinafter and illustrated in the accompanyingdrawings.

In the drawings:

FIG. 1 shows, in a cross-section through its optical axis, a knownelectrowetting lens in a non-activated state;

FIG. 2 shows such a lens in an activated state;

FIG. 1 shows an electrowetting module constituting a variable focuslens. The element comprises a first cylindrical electrode 2 forming acapillary tube, sealed by means of a transparent front element 4 and atransparent rear element 6 to form a fluid chamber 8 containing twofluids. The electrode 2 may be a conducting coating applied on the innerwalls of a tube.

In this embodiment of the electrowetting module the two fluids consistof two non-miscible liquids in the form of an electrically insulatingfirst liquid A, currently, for example a silicone oil or an alkane, andan electrically conducting second liquid B, currently, for example,water containing a salt solution. The first fluid A has a higherrefractive index than the second fluid B.

The first electrode 2 is a cylinder of inner radius typically between 1mm and 20 mm. This electrode is formed of a metallic material and iscoated by an insulating layer 10, formed for example of parylene. Theinsulating layer has a thickness of between 50 nm and 100 μm. Theinsulating layer is coated with a fluid contact layer 12, which reducesthe hysteresis in the contact angle of the meniscus 14, i.e. theinterface between the fluids A and B, with the cylindrical wall of thefluid chamber. The fluid contact layer is preferably formed from anamorphous fluorocarbon such as Teflon™ AF1600 produced by DuPont™. Thefluid contact layer 12 has a thickness between 5 nm and 50 μm. It isalso possible that one layer is used with both insulating andhydrophobic properties.

A second electrode 16 is arranged at one side of the fluid chamber, inthis case, adjacent the rear element 6. The second electrode is arrangedsuch that at least one part thereof is in the fluid chamber, so that theelectrode can act on the second fluid B.

The two fluids A and B are non-miscible so as to tend to separate intotwo fluid bodies separated by a meniscus 14. When no voltage is appliedbetween the first and second electrodes, the fluid contact layer 12 hasa higher wettability with respect to the first fluid A than with respectto the second fluid B. FIG. 1 shows this lens configuration, i.e. thenon-activated state of the electrowetting lens. In this configuration,the initial contact angle θ between the meniscus and the fluid contactlayer 12, measured in the fluid B, is larger than 90°. Since therefractive index of the first fluid A is larger than the refractiveindex of the second fluid B, the lens formed by the meniscus, herecalled meniscus lens, has a negative power in this configuration.

Due to electrowetting, the wettability by the second fluid B variesunder the application of a voltage between the first electrode and thesecond electrode, which tends to change the contact angle. FIG. 2 showsthe lens configuration if such a voltage from a source 17 is supplied tothe lens, i.e. if the lens is in the activated state. In this case thevoltage is relatively high, for example between 150V and 250V and themeniscus has now a convex shape, with respect to the body of theelectrolyte. The maximum contact angle θ between the meniscus and thefluid contact layer 12 is, for example of the order of 60°. Since therefractive index of fluid A is larger than fluid B, the meniscus lens 1in this configuration has a positive power and it focuses an incidentbeam b in a focal spot 18 at a certain distance d from the lens.

For further details about the construction of the variable focus lensreference is made to international patent application no. IB03/00222. Azoom lens, which comprises at least two independently controllableinterfaces between a higher refractive index liquid and lower refractiveindex fluid, is described in the European patent application no.02079473.1 (PHNL021095).

In practice there is a need to lower the freezing point of theconducting liquid of an electrowetting module so that the module willstill work at temperatures of for example −20° C.

Of course, the electrowetting properties of the module should bemaintained at such low temperatures.

A known method to lower the freezing point of a liquid is to dissolve asalt in the liquid. The theoretical freezing point depression reached bya given amount of salt in moles of ions per kilogram liquid canapproximately be determined form the equation:ΔT _(f) =K _(f) c _(m)wherein ΔT_(f) is the change in freezing point, K_(f) is the freezingpoint depression constant, and c_(m) is the molar concentration of ionsof the solution.

It follows from this equation that large concentrations of salt areneeded to effect a sufficient drop in the freezing point. Nevertheless,by dissolving too much salt in the liquid, the density of the liquid mayincrease too much, and as a result, the required density matchingbetween the conductive and non-conductive liquids can not always be met.

Further, dissolving salt in a liquid may give rise to a change inrefractive index. When the refractive index of the conductive liquidincreases upon the addition of a salt, the refractive index between thisliquid and the non-conductive liquid decreases (provided that thenon-conductive liquid has a higher refractive index than the conductingliquid), which results in an undesirable decrease in optical power rangeof an electrowetting lens.

In an electrowetting lens the optical power of the lens depends on thecurvature of the meniscus and the difference in refractive indicesbetween the conductive and non-conductive liquids, as can be seen in thefollowing equation:

$S = \frac{n_{1} - n_{2}}{r}$

Wherein S is the optical power of the meniscus lens, r the radius ofcurvature of the meniscus, n₂ the refractive index of the non-conductiveliquid and n₁ the refractive index of the conductive liquid.

The non-conductive liquids currently used in electrowetting lenses (e.g.alkanes or silicone oils) have a refractive index (n=1.37–1.43) that isonly slightly larger than the refractive index of the currently usedconductive liquids (e.g. water, n=1.33).

According to the present invention an amount of a compound, preferablyan inorganic salt, sufficient for lowering the freezing point of thefirst conductive fluid to approximately −20° C., is added to said firstconductive fluid, whereby said salt preferably contains cations having alow molecular weight of below 50 u.

This measure will keep the refractive index and density for theconductive liquid low, whilst the other requirements for the liquid,such as high transparency, non-miscibility with the other liquid orfluid, and a good electrowetting behavior can still be satisfied.

Examples of some liquids and their freezing point depression constantsto obtain a freezing point of −20° C. are given in Table 1.

TABLE 1 Solvent Melting point (° C.) K_(f)(° C./m) c_(m) for −20° C.Water 0.000 1.858 10.8 Acetic acid 16.60 3.59 10.2 Benzene 5.455 5.0655.0 Camphor 179.5 40 5.0 Cyclohexane 6.55 20.0 1.3

In table 1 is c_(m) the molar concentration of the ions in the solutionto obtain a freezing point below −20° C. Since each salt molecule givesrise to at least two ions, the required amount of salt dissolved in theliquid is smaller than cm by at least a factor of two. When, for exampleLiCl is used as the inorganic salt in the conductive liquid, a solutionof 5.4 M LiCl is required to lower the freezing point of water to −20°C.

From experiments it follows that for the most suitable fluids used inthe electrowetting module a solution having a concentration of 4 M isrequired to lower the freezing point to −20° C. Although here inorganicsalts are mentioned also organic compounds, such as for instanceethyleenglycol, ethanol or methanol may be used. Again experiments showthat 4 M molecules of organic materials are required to lower thefreezing point to −20° C.

It is observed that it is known from US 2003/0095336 A1, paragraph[0063], to use an alcohol, glycerin, silicone or mineral oil to lowerthe freezing point of a fluid body of an optical element, morespecifically a lens. This lens is nevertheless not based onelectrowetting; the description, further, does not mention anyconcentration, nor an indication about the advantages of certainconcentrations of the compounds to be used.

As has been mentioned before, it is preferred that the cationsconstituting the inorganic salt have a low molecular weight, preferablybelow 40 u.

A preferred group of compounds to be used in the present inventionconsist of LiCl, NH₄Cl en NaCl, whereof LiCl is the most preferredcompound, to be used with water as the conductive solvent. Fluorinesalts can also be used, provided that they give the desired freezingpoint depression.

Yet another preferred group of compounds consists of salts with formateor acetate as the cations, e.g. ammonium formate or lithium formate.

Further, it can be necessary to lower the freezing point of the second,non-polar fluid also. Preferred compounds to be used in this fluid arecarbon tetrabromide, trichlorobenzene, naphthalene and biphenyl.

When use is made of an inverted lens in an electrowetting opticalmodule, the conductive liquid has a higher refractive index than thenon-conductive liquid. In such a case, an increase in the refractiveindex of the conductive liquid is desired. This can be obtained when asalt is used whereof the cation has a high molecular weight, i.e. higherthan 40 u, such as for example Cs₂WO₄, whereof an aqueous solution has arefractive index n=1.482. When combined with a non-conductive liquidhaving a low refractive index, such as a silicon oil whereof n=1.37, anoptical electrowetting module can be made, which has a large opticalpower.

To allow density matching of the conductive liquid with thenon-conductive liquid in an inverted lens, it is nevertheless requiredthat the conductive liquid should be mixed with a compound having alower density, to match with the density of the non-conductive liquid,usually a silicone oil.

Although the description has been limited to an electrowetting lens asan example of an optical electrowetting module, the invention is not inany way limited to such a lens. The invention may be used in any opticalelectrowetting module, such as a variable-focus lens, a zoom lens, adiaphragm, a filter and a beam deflector, but also in a mechanicalelectrowetting module as a pump and a motor.

1. An electrowetting module comprising a fluid chamber, containing atleast a first body of a first conducting and/or polar fluid and a secondbody of a second non-conducting and/or non-polar fluid, the two bodiesbeing separated by an interface, and means for exerting a force on atleast one of the bodies to change the position and/or shape of theinterface, characterized in that the second fluid body comprises adissolved or mixed compound being insoluble in or immiscible with thefirst fluid body, and/or the first fluid body comprises a dissolved ormixed compound being insoluble in or immiscible with the second fluidbody, said amounts of compounds being sufficient for lowering thefreezing point of the respective fluids to below −20° C.
 2. Anelectrowetting module as claimed in claim 1, wherein the amount of thedissolved compound is sufficient for lowering the freezing point of therespective fluid to below −30° C., preferably to −40° C.
 3. Anelectrowetting module as claimed in claim 1, wherein the concentrationof said dissolved compound in said fluid body is at least 4 M,preferably 6 M.
 4. An electrowetting module as claimed in claim 1,wherein said compound being dissolved in or mixed with the first body offluid is an inorganic salt containing cations having an atomic weightbelow 50 u, preferably below 40 u.
 5. An electrowetting module asclaimed in claim 4, wherein said inorganic salt is a chlorine salt,preferably lithium chloride, ammonium chloride or sodium chloride, morepreferably lithium chloride.
 6. An electrowetting module as claimed inclaim 1, wherein said compound being dissolved in or mixed with thefirst body of fluid is an organic compound, preferably selected fromamong methanol, ethanol and ethylene glycol.
 7. An electrowetting moduleas claimed in claim 1, wherein said compound being dissolved in or mixedwith the second body of fluid is selected from carbon tetrabromide,dibromobenzene, tribromobenzene, dichlorobenzene, trichlorobenzene,naphthalene and biphenyl.
 8. A module as claimed in claim 1, configuredas an optical component, the first and said second fluid bodies havingdifferent refractive indices, wherein the compound added to said firstfluid has a refractive index difference increasing effect.
 9. A moduleas claimed in claim 8, wherein the first fluid body is electricallyconducting and/or polar, and the second fluid body is electricallynon-conducting and/or non-polar, the module being provided with meansfor exerting an electric force to change the position and/or shape ofthe meniscus-shaped interface.
 10. A module as claimed in claim 8,wherein the difference in refractive index is from 0.01 to 0.3,preferably from 0.1 to 0.2; the refractive index of one of the bodiesbeing greater than 1.4, preferably greater than 1.5.
 11. A module asclaimed in claim 8, wherein said first and said second fluid bodies showa substantially similar density.
 12. A module as claimed in claim 8,provided with means for exerting a pressure to change the positionand/or shape of the interface.